Enigmatic subglacial bedforms ("glacial curvilineations") of the Dobrzyñ moraine plateau, Poland: genesis and glaciodynamic implications

Category Environmetal Geology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Lesemann, Jerome-Etienne۱; Piotrowski, Jan A.۱; Wysota, Wojciech۲
Holding Date 07 October 2008

The glacial landscape of the Dobrzyñ moraine plateau in central Poland contains a complex suite of enigmatic bedforms. These forms have previously been termed drumlins, though there is little agreement on the genesis of this landscape. However, the synoptic perspective offered by digital terrain models reveals a suite of landforms far different from classical drumlins. The landscape consists of elongate and continuous sediment ridges. We propose the term "glacial curvilineations" to distinguish these forms from classical drumlins and to emphasize the uniqueness of these bedforms. To our knowledge, such forms and their regional patterns have not been previously recognized in the literature of glaciated environments. Consequently, formative mechanisms and the glacier dynamics associated with this landscape remain enigmatic.
The Dobrzyñ moraine plateau consists of a gently southward-sloping surface of subdued relief and composed of interbedded glaciofluvial sediments and diamictons. Extensive channels exhibiting convex longitudinal profiles are eroded in the Moraine Plateau and form a regional anabranched network teminating at an ice margin marked by coalesced glaciofluvial fans. More enigmatic landforms also occur. These consist of groups of parallel, sinuous ridges and troughs developed within, and aligned with the channels. Ridges can be as long as 8-14 km with undulating crestlines. Ridge spacing is 60-120 m and intervening troughs exhibit localized overdeepenings occupied by lakes that are sometimes crescent-shaped. Field exposures in ridges reveal sediment beds truncated by the ridge surface.
The landscape of the Dobrzyñ moraine plateau is dominantly erosional. Spatial association of channels and ridges, and their common geomorphic attributes suggest a common genesis. Both channels and ridges are the products of subglacial meltwater erosion. Channels are tunnel valleys, eroded by pressurized subglacial flows. Ridges are erosional remnants produced by longitudinal vortices within the channel-forming flows. Initial channel development determines the lateral extent of individual ridge fields. The remarkable parallelism of ridges reflects spacing of coherent turbulent structures within the subglacial flow. Channel margins also control ridge sinuosity by dictating both the path and curvature of longitudinal vortices. Extensive outwash fans downflow of channels are depositional end-members of the erosion within channels and further support the interpretation of meltwater erosion.
This landscape also sheds light on glacier hydrology. The occurrence of discontinuous channels and the sudden appearance of channel heads suggest point-sources of meltwater such as subglacial cavity drainage and/or supraglacial ponds draining subglacially through moulins and crevasses. This latter possibility has recently been documented in Greenland. The landscape of the Dobrzyñ moraine plateau may be the geomorphic record of such supraglacial-subglacial connections.